1. Field of the Invention
The invention relates to an electric furnace, and more particularly to a d.c. arc furnace having a cooled bottom electrode and including at least one connecting piece located below the bottom of the furnace vessel.
2. Description of the Prior Art
A furnace of the above-noted type is known, for example, from Swiss Patent Specification No. 452,730.
Progress in the development of semi-conductor components in recent years has given an impetus to an increasing use of direct-current arc furnaces in the iron and steel industry for smelting, predominantly for smelting electric-furnace steel.
The construction and mode of action of direct-current arc furnaces are known, for example, from the journal "Stahl und Eisen", 103 (1983) No. 3, of Feb. 14th, 1983, pages 133 to 137.
For optimizing the electrical or thermal conditions, it has proven to be advantageous in a direct-current arc furnace to form the arc between one or more electrode(s) located above the melting material and the melting material itself. As the return line of the direct current, at least one electrode in the bottom of the furnace and in contact with the melt, namely the bottom electrode, is provided.
The bottom electrode is exposed to continuous, very high thermal stresses, for which materials having a high softening and melting point, for example graphite, are suitable. However, when carbon electrodes are used, the melt is carburized on the one hand. This is undesirable, however, in particular in the production of low-carbon steels. On the other hand, the carbon electrode is consumed, whereby the furnace bottom is weakened and the electric power transfer can be adversely affected.
According to the solution proposed by the above-noted Swiss Patent Specification No. 452,730, bottom electrodes are used, but have a zone also having the same chemical contents as the melt itself in contact with the melt. In this case, cooling takes place in the end zone, facing away from the furnace vessel, of the bottom electrode by convection with air, this end zone consisting of a metal having good heat-conducting and current-conducting properties, for example of copper. This is a so-called two-component bottom electrode.
On the one hand, this air-cooled two-component bottom electrode avoids, in the event of a break-out of the furnace hearth, the possibility of molten metal coming into contact with the liquid of a cooling arrangement or with current-bearing components of the bottom electrode below the furnace vessel bottom, and thus from the start eliminates the risk of unforeseen serious consequences. On the other hand, a relatively weak cooling effect must be accepted, which is by no means up to the demands which a continuously operating bottom electrode in industrial use must meet, namely for the following reasons:
The operation of an arc furnace is essentially characterized by five process stages:
______________________________________ the charging phase no power, low temperature the fusion phase high power, high tempera- ture the refining or low power; high tempera- purification phase ture the tapping phase no power; high tempera- ture the non-productive no power; medium-to-low time phases temperature ______________________________________
In particular the dissipation heat, generated in the fusion phase by the current, causes a greater heat flux in the bottom electrode, in particular in the direction of the furnace vessel bottom. Accordingly the intensity of the heat arising can vary within a relatively wide range between the charging phase and the refining or purification phase. However, this also means that the temperatures prevailing in the cooled zone of the bottom electrode can likewise vary within a relatively wide range. With a constant length of the bottom electrode, the differing heat flux in the bottom electrode can produce varying temperature differences between its cooled zone and its zone in contact with the melt. With more heat arising, however, there is no greater temperature difference, since the electrode cannot be warmer on the inside than the temperature of the melt, or, in other words, more heat can be transported only if the bottom electrode becomes shorter, that is to say melts off.
If, as is the case in a wide range with air cooling, the temperature of the cooling surface is substantially higher with more heat arising than with less heat arising, the greater quantity of heat can only be removed if the length of the bottom electrode is shortened even more, that is to say even more of the bottom electrode melts off. It follows that the change in position of the liquid/solid boundary layer between the melt and the bottom electrode extends over a relatively great length, as viewed in the axial direction. This change in position can manifest itself, on the one hand, by the bottom electrode "growing into" the melt or, as already stated, in a melting-off process in the direction of the furnace vessel bottom.
The process described above impairs the durability of the bottom electrode to a considerable extent and leads to premature destruction of the refractory lining, surrounding the bottom electrode, of the furnace vessel bottom. To ensure that the bottom electrode is operable at all under such unfavorable operating conditions, it must be correspondingly oversized. This in turn has an adverse effect on the dissipation power.
Furthermore, the cooler output must be adapted to the operational requirements. On the one hand, this can be increased by oversizing an air cooler. However, this would give unsatisfactory results in the long run.
On the other hand, liquid cooling would be outstandingly suitable for cooling a bottom electrode. In this case, however, appropriate protective measures must be taken to prevent liquid metal from coming into contact with cooling liquid.
Swiss Patent Specification No. 595,526 discloses a bottom electrode with liquid cooling, in which, however, the liquid cooling device extends through the furnace vessel bottom, for a considerable distance into the furnace interior.
This arrangement involves a considerable risk of liquid metal coming into contact with cooling liquid, in particular if melt has infiltrated the refractory lining of the furnace vessel bottom.